In order to better evaluate nondestructive inspection (NDI) technologies, it is advantageous to develop real, as opposed to simulated, fatigue-cracks in test structures, such as aircraft test structures, to expedite review of such NDI technologies. However, fatigue-cracks may be understood to be difficult and time consuming to initiate, and prior methods of initiation often do not result in satisfactory results.
In order to initiate a fatigue-crack, a starter notch may be formed in the structure. Saw cuts formed with a blade were initially evaluated for the starter notches, but are too large to be useful, particularly as a starter notch often needs to be on the order of about 0.005 inch in width, and about 0.010 inch penetration into the workpiece.
More recently, a process that is often used to obtain very small starter notches is electric discharge machining (EDM). This approach has been widely used for producing starter notches in aircraft structures such as aluminum, titanium, steel, and other aircraft metal.
However, it has been recently determined that when attempting to produce a very small and tight fatigue-crack using the EDM starter notches, the fatigue-crack grows in directions that are not along the major axis of the EDM starter notch. While not being bound to a particular theory, one explanation for the cause of this phenomenon is that the EDM process creates small micro cracking along the edges of the EDM notch and when fatiguing stress is applied to the EDM notch, the micro cracks grow along the orientation of the micro cracking and not the major axis of the EDM notch. Another theory is that the high temperature of the electrical arc induces a change is the morphology of the metal structure. As such, methods and devices are needed to produce started notches in fatigue test structures which will result in a more natural growing of fatigue-cracks.